1. Technical Field
The invention relates to a process for preparing a high-impact vinylaromatic polymer composition, in other words a composition comprising a matrix of vinylaromatic polymer and particles of rubber.
According to the prior art, the particles of rubber can be in various morphologies. The following morphologies may be obtained:
the "salami"-type morphology, which means that the rubber particle contains a number of occlusions, generally substantially spherical but not concentric, of vinylaromatic polymer, PA1 the "labyrinth"-type morphology, which means that the rubber particle contains a number of elongated, curved, generally aspherical and generally asymmetric occlusions of vinylaromatic polymer, PA1 the "onion"-type morphology, which means that the rubber particle is substantially spherical and contains, concentrically with respect to itself, a number of occlusions of vinylaromatic polymer contained within one another, PA1 the "capsule"-type morphology, which means that the rubber particle, which is generally substantially spherical, contains a single occlusion of vinylaromatic polymer. PA1 Echte et al., J. Sci. Ind. Res. 40, 659 (1981), PA1 EP 48389 PA1 Echte, Rubber Toughened Plastics, C. Keith Riew Editor, American Chemical Society, Washington, 1989. PA1 tert-butyl isopropyl monoperoxycarbonate, PA1 tert-butyl 2-ethylhexyl monoperoxycarbonate, PA1 dicumyl peroxide, PA1 di-tert-butyl peroxide, PA1 1, 1-di (ttert-butylperoxy) cyclohexane, PA1 1, 1 -di (tert-butylperoxy)-3,3, 5-trimethyl- cyclohexane, PA1 tert-butyl peroxyacetate, PA1 cumyl tert-butyl peroxide, PA1 tert-butyl peroxybenzoate and PA1 tert-butyl peroxy-2-ethylhexanoate. PA1 (SFR): the number of moles of stable free radical, PA1 F.sub.SFR : the functionality of the stable free radical, i.e., the number of sites on the same stable free radical molecule which exhibit the stable free radical state, PA1 (INIT): the number of moles of free-radical initiator, PA1 F.sub.INIT : the functionality of the free-radical initiator, i.e., the number of free radicals which each molecule of initiator is capable of generating, and PA1 (ELAST): the number of moles of elastomer; then, generally, the ingredients required for the heat treatment are introduced such that ##EQU1## PA1 N-tert-butyl-1-phenyl-2-methylpropyl nitroxide, PA1 N-tert-butyl-1-(2-naphthyl)-2-methylpropyl nitroxide, PA1 N-tert-butyl-1-diethylphosphono-2, 2-dimethyl-propyl nitroxide, PA1 N-tert-butyl-1-dibenzylphosphono-2, 2-dimethylpropyl nitroxide, PA1 N-phenyl-1-diethylphosphono-2, 2-dimethyl-propyl nitroxide PA1 N-phenyl-1-diethylphosphono-1-methylethyl nitroxide PA1 N-(1-phenyl-2-methylpropyl)-1-diethyl-phosphono-1-methylethyl nitroxide, PA1 4-hydroxy-2, 2, 6, 6-tetramethyl-1-piperidinyloxy, PA1 4-oxo-2, 2, 6, 6-tetramethyi-1-piperidinyloxy, PA1 2, 4, 6-tri-tert-butylphenoxy. PA1 per 100 parts by weight of vinylaromatic monomer PA1 from 2 to 35 parts by weight of carrier rubber and PA1 from 0 to 50 parts by weight of solvent. PA1 tert-butyl isopropyl monoperoxycarbonate, PA1 tert-butyl 2-ethylhexyl monoperoxycarbonate, PA1 dicumyl peroxide, PA1 di-teri-butyl peroxide, PA1 1, 1-di (tert-butylperoxy)cyclohexane, PA1 1, 1-di(tert-butylperoxy)-3, 3, 5-trimethylcyclohexane, PA1 tert-butyl peroxyacetate, PA1 cumyl tert-butyl peroxide, PA1 tert-butyl peroxybenzoate, PA1 tert-butyl peroxy-2-ethylhexanoate. PA1 rubber particle morphology: transmission electron microscopy on fine sections stained with osmium tetroxide, PA1 median size of the rubber particles: sedimentation granulometry (CAPA 700) after dissolution of the matrix in methyl ethyl ketone.
The salami, labyrinth and onion morphologies can be termed "multi-occlusion" morphologies. They are generally substantially larger than is the capsules.
These morphologies affect the impact properties and the gloss of vinylaromatic polymer compositions. The influence of this morphology on the gloss, in increasing order of gloss, is as follows: salami, then labyrinth or onion, then capsule.
In general, the greater the occlusion content of a particle the larger it is. This is why the rubber particles generally increase in size in the following order: capsule, then labyrinth or onion, then salami. It is for this reason that it is possible to consider the possibility of enhancing the gloss of a high-impact vinylaromatic polymer composition by reducing the size of the particles of rubber it contains.
Depending on the target properties of the material, especially its impact resistance and gloss, one may be led to combine within the same material particles having different morphologies and, for example, to adjust the capsules/multi-occlusions ratio. In particular, a material whose mass of rubber particles consists predominantly of capsules and in which the remainder of the mass of particles, for example from 5 to 40%, consists of multi-occlusions exhibits a good tradeoff between impact and gloss.
If the desire is to obtain a particularly glossy high-impact vinylaromatic copolymer or polymer, one may be led to research the preparation conditions which lead exclusively to the capsule morphology.
2. Description of Related Art
According to the prior art, the morphologies other than the salami morphology, in other words the labyrinth, onion or capsule morphologies, cannot be obtained by making use of a conventional polydiene but only by polymerizing the vinylaromatic monomer in the presence of a styrenebutadiene copolymer. In particular, in accordance with the prior art, the capsule morphology would only be obtainable by polymerizing styrene in the presence of a styrene-butadiene copolymer comprising at least 15 and 15 preferably 40% styrene. This teaching results in particular from the following documents:
However, such copolymers are expensive and, moreover, must be introduced into the polymerization medium in higher concentrations than a conventional polydiene for a given level of polydiene in the final high-impact vinylaromatic composition. Such amounts of copolymer are, moreover, liable to require longer periods of solubilization within the styrene prior to polymerization. For this reason it is desirable to know how to obtain a capsule, onion or labyrinth morphology while making use of a conventional, less expensive rubber which can be introduced in a smaller quantity into the polymerization medium.
The desired capsules/multi-occlusions ratio for a given material can, for example, be obtained by mixing defined amounts of a high-impact vinylaromatic polymer containing only capsules and a high-impact vinylaromatic polymer containing only particles of the multi-occlusion type.
The patent application EP 0726280 teaches that it is possible to prepare a high-impact polystyrene by polymerizing styrene in the presence of a rubber and a stable free radical. However, in the practical examples of this document, the final material does not include any capsule particles. The practical examples of that document teach that the presence of the stable free radical during polymerization may lead to an increase in the size of the rubber particles within the end material.